A head/disk tester is an instrument that is used for testing the characteristics of magnetic heads and disks, such as signal-to-noise ratio, track profile, etc. The tester simulates motions of the head with respect to the disk that occur in an actual hard disk drive during operation. A tester comprises a mechanical component, commonly referred to as a spinstand, that performs movements of the head with respect to the disk, and an electronic component, that is responsible for measurement, calculation, and analysis of the measured signal.
Examples of prior art spinstands for a head and disk tester include the Guzik V2002 XY-positioning spinstand and the Guzik S-1701B Micro Positioning spinstand, both of which are available from the assignee of the present disclosure, Guzik Technical Enterprises, 2443 Wyandotte Street, Mountain View, Calif. 94043, USA.
A read/write head is usually incorporated into a head gimbal assembly (HGA), such as shown in FIG. 1. The basic components of an HGA 100 are a head 102, an elongated load beam 104, a tooling hole 106, a base plate 108 having a planar major surface extending between a first edge and a second edge, a boss hole 110 with an angled surface 110A, and a elongated flex circuit support sheet element 112 with an array of electrically conductive pads 118. The boss hole 110 passes through base plate 108 and is characterized by a radius R about an HGA mounting axis perpendicular to the planar surface with center point CP. The boss hole 110 and the tooling hole 106 (sometimes) are used for orientation of the HGA in the X-Y plane. The angled surface 110A of the boss hole 110 is used for clamping the HGA to an HGA support assembly associated with a spinstand. The flex circuit sheet element 112 is used to support electrical connections of the head of the HGA, by way of pads 118, to an external head preamplifier (not shown). Generally, the base plate 108 and load beam 104 are relatively stiff compared to the flex circuit sheet element 112.
In order to test a head with a spinstand, an HGA is loaded to the HGA support assembly associated with the tester. The HGA is mechanically coupled to a corresponding component of the spinstand, and electrically connected to spinstand preamplifiers. To make these operations possible, an alignment of the HGA relative to the spinstand is carried out. After testing, the tested head is removed from the tester.
In the prior art, these steps may be performed by a human operator. Alternatively, some or all of these steps can be automated. Automation is particularly useful in a manufacturing environment, as automation can perform loading faster than a human operator and thus can lead to a greater throughput of heads. Modern heads are particularly susceptible to electrostatic damage. Using automation to perform HGA loading, instead of human operator, helps to avoid such damage and to reduce the loss of the heads.
Various methods and apparatus for automatic loading the HGA to a tester are known in the prior art and are described, for example, in U.S. Pat. No. 7,520,047 “Method and apparatus for loading a read/write head to a spinstand”, U.S. Pat. No. 7,542,868 “Head gimbal assembly loader”, U.S. Pat. No. 7,529,635 “Method and apparatus for head gimbal assembly testing”, U.S. Pat. No. 8,176,794 “Unmounted head gimbal assembly clamping plate”, U.S. Pat. No. 8,873,200 “Spinstands for testing a head gimbal assembly”, U.S. Pat. No. 8,611,048 “Apparatus and method for receiving and positioning a read/write head to a disk for testing and method of removing a tested read/write head from a test apparatus”.
A tester with automatic loading of an HGA, known in the prior art, usually contains a loading area, a precising (or “alignment”) area and a test area. A transporter, incorporated in the tester, enables transfer of the tested HGAs from the loading area to the precising area, from the precising area to the test area and, after testing, from the test area back to the loading area. In the loading area, an operator loads and unloads HGAs using some HGA containers. In the precising area, the HGA is aligned with the disk for eventual testing. The components of the precising area are purportedly accurately aligned with corresponding components of the test area, so that the alignment performed at the precising area is retained when the HGA is moved to the test area. In the test area, an electrical connection of the HGA with the preamplifier is established and a dynamic test of the read/write head in association with a disk is performed.
Typically, in the prior art, at a planar surface in an X-Y plane in the precising area, a boss hole pin and a front alignment pin are erected. Both pins are tapered. When an HGA to-be-tested is lowered along the Z direction at the planar surface of the precising area, the HGA's boss hole 110 slips over the boss hole pin and the tooling hole 106 slips over the front alignment pin. As the HGA travels downward, the taper on the pins pulls the boss hole 10 and the tooling hole 106 into their proper positions. The achieved alignment is maintained while the HGA is transferred to the test area. After the transfer to the test area has been carried out, the flex circuit pads 118 of the HGA are generally in the vicinity of the preamplifier terminals and the mechanical structure of the tester attempts to establish an electrical connection between the HGA and the preamplifier terminals, and the preamplifiers.
There are two straight lines that characterize the geometry of an HGA: (a) an axis of symmetry of the HGA's base plate 108 that goes through the centers of the boss hole 110 and the tooling hole 106, and (b) a flex axis that passes through the center point CP of the boss hole 110 and through the array of electrically conductive pads 118 of the flex circuit sheet element 112. The angle between these two lines is not the same for all HGAs—it varies from one type of HGA to another. Accordingly, the position of the pads 118 relative to the axis of symmetry of the HGA's base plate 108 varies—the pads 118 of the flex circuit sheet element 112 of different HGAs are scattered around some nominal (proper) position.
The structure that establishes electrical connection between the pads of array 118 and preamplifier terminals, is designed in such a manner that the terminals of the preamplifier come into contact with the pads 118 of the flex circuit sheet element 112 of an “average” HGA. The pads 118 of a real HGA, which is to be tested, are generally displaced from the “average” position. If the displacement is small enough, it does not hinder the establishment of an electrical connection. If the displacement exceeds a certain value, which is typically of the same order of magnitude as a pad size, then the pads 118 of the HGA's flex circuit sheet element 112 and the preamplifier terminals are spaced apart and are thus mis-aligned so that the needed electrical connection cannot be established for the subject HGA. In this situation, the test fails and, typically, the failed “not-tested” HGA (and its head) is marked as unfit for use.
In the prior art, the “two points alignment” of the HGA, which is performed by aligning the positions of the boss hole 110 (the first point) and the tooling hole 106 (the second point), and which was suggested in the above-cited patents, aligns only the position of the HGA's base plate 108. After that limited alignment, the HGA becomes fixed in the X-Y plane and further alignment is impossible. Any possible shift of the circuit pads of array 118 in relation to the preamplifier terminals however remains unchanged, so that the pads cannot be brought into contact with the preamplifier terminals. This is particularly important since the circuit sheet element 12 relatively compliant/flexible compared to the relatively stiff base plate 108 and load beam 104. If the load beam 104 and base plate 108 are fixed, as in the prior art, the pad array 118, at the distal end of the flexible circuit sheet element is simply not known/controlled. Attempts to establish an electrical connection fail often enough for this reason, so that significant numbers of possibly good, but untested, heads are baselessly rejected as flawed, because of their inability to connect the HGA to preamplifier terminals, and are erroneously discarded. This is an important disadvantage of prior art HGA loaders.
There are several ways suggested in the prior art, to overcome the adverse effect of the positional errors of the HGA flexible circuit sheet element upon test results (see, for example, the U.S. Pat. No. 7,529,635, entitled “Method and apparatus for head gimbal assembly testing”). It has been suggested that the area of preamplifier terminals be widened, to enable a higher likelihood of successful interconnection with the flex circuit pads 118. By using contacts with an area that covers the tolerance range of flex circuit location, one may ensure that the flex circuit pads 118 touch the preamplifier terminals, when they are pressed together.
Another suggestion has been to terminate each of the preamplifier conductors, which are to contact the same flex circuit pad, with two pogo pins. The use of two pogo pins for one conductor permits the flex circuit pad to contact one of them, even when the flex circuit 112 is not optimally aligned. This would increase the variance permitted in the positioning of the flex circuit 112. However, for increasingly needed high density connector pad arrays, it is not possible to accommodate error tolerances in the position of the pad arrays 118.
The trend of increasing track density on a disk, has led to new methods of magnetic writing, such as those encompassing preheating of the media. Such new techniques have introduced new components (a heater, a laser and so on) to read/write heads, so that the number of head inputs/outputs is increased. This causes a corresponding increase of the number of pads on an HGA flex circuit 112. Some manufacturers have begun to produce HGA's with flex circuit pads 118 arranged in several rows (see FIG. 2B and FIG. 2C, for example). For this reason, the described suggestions cannot be used in the needed contemporary HGA loaders, and the problem of elimination (or at least reduction) of the flex circuit sheet element 112 positioning-caused errors, remains urgent.